This invention relates to the molding and application of protective caps to microelectronic semiconductor chips on a wafer scale as opposed to application on an individual chip basis. More particularly the invention relates to the molding and application of protective caps to semiconductor chips incorporating Micro Electro Mechanical Systems (MEMS). However the invention is not limited to MEMS applications.
Semiconductor chips are normally packaged in a protective layer or layers to protect the chip and its wire bonds from atmospheric and mechanical damage. Existing packaging systems typically use epoxy molding and thermal curing to create a solid protective layer around the chip. This is normally carried out on individually diced chips bonded to lead frames and so must be done many times for each wafer. Alternative methods of packaging include hermetically sealed metal or ceramic packages and array packages, such as ball grid array (BGA) and pin grid array (PGA) packages. Recently wafer scale packaging (WSP) has started to be used. This is carried out at the wafer stage before the chips are separated. The use of molding and curing techniques subjects the wafer to both mechanical and thermal stresses. In addition the protective cap so formed is a solid piece of material and so cannot be used for MEMS devices, since the MEMS device would be rendered inoperable by the polymer material. Existing packaging systems for MEMS devices include thematically sealed packages for individual devices, or use silicon or glass wafer scale packaging, both of which are relatively high cost operations.
In one broad form the invention provides a method of applying a plurality of caps to a plurality of microfabricated devices at the wafer stage, the method including:
a) providing a wafer having a plurality of microfabricated devices;
b) providing a plurality of first hollow molded caps, one cap for each of the devices or a predetermined group of devices, each cap having a central portion and a perimeter wall extending from the perimeter edge of the central portion;
c) applying the first caps substantially simultaneously to one side of the wafer with each cap overlying part or all of a device or a predetermined group of devices with the free edge of the perimeter wall contacting the wafer;
d) bonding first the caps to the wafer; and
e) separating the wafer into individual packages.
The wafer is preferably formed of a semiconductor, such as silicon.
The devices may be semiconductor devices or other microfabricated devices, such as micro mechanical systems, MEMS, Micro Optical Electro Mechanical Systems (MOEMS), passive elements such as capacitors, resistors, inductors, conductors and the like or any combination of the foregoing.
The individual packages are preferably separated by removing material from between adjacent packages.
When the wafer is a semiconductor, the material between adjacent caps is preferably removed by a deep plasma etch. The etch is preferably applied from the cap side, so that the caps act as a mask for the etch.
Caps may be applied to the top and bottom of the wafer and the semiconductor material may be removed by a deep plasma etch from the bottom of the wafer.
A second plurality of caps may be applied to a second side of the wafer, before after or simultaneously with the first plurality of caps are applied to the wafer.
The caps may be bonded to the wafer using a glue, bonding agent or merely by pressing the softened caps against the wafer.
The material of the cap may be chosen to absorb infrared radiation to enable infrared heating of the cap material. Preferably the material of the cap absorbs infrared radiation within the wavelength range of about 1000 nm to about 5000 nm.
Preferably the cap is a thermoplastic material.
The wafer may be separated into packages each having a single cap attached to one side thereof.
The caps may be joined to each other by cap material and the devices may be separated by removing both the cap material between adjacent caps and the wafer material.
The cap material between adjacent caps and the material may be removed by mechanical or thermal means, such as sawing or laser ablation.
Where the caps are joined together on application to the wafer, the cap material between adjacent caps may be removed by an oxygen plasma etch.
In another broad form the invention provides an array of hollow caps, each of the caps including:
a central portion having a peripheral edge or edges; and
a peripheral wall or walls extending away from the central portion with the free end or ends of the peripheral wall or walls generally lying in a plane remote from the central portion to define a mouth;
the array having the caps:
a) in a common orientation;
b) the mouths of the caps in a common plane; and
c) at a spacing to enable the array of caps to be placed on a wafer including a plurality of microfabricated devices with the mouths of the caps contacting the wafer and each cap overlying part or all of one of the devices or a predetermined group of devices.
Preferably the material of the cap absorbs infrared radiation and more preferably absorbs infrared radiation within the wavelength range of about 1000 nm to about 5000 nm.
Each cap may have least one aperture in the central portion.
Each cap may have one or more walls extending from the central portion for bonding with the wafer and/or the microfabricated device to define a corresponding channel therebetween. Each channel preferably aligns with a corresponding aperture extending through the thickness of the wafer. Additionally, preferably each cap has at least one aperture extending through the central portion to communicate a respective channel with the outside environment, whereby there is provided a fluid communication from the outside environment through the cap and the aperture in the wafer to the other side of the wafer.
Preferably the caps are formed of a thermoplastic material.
The caps may be joined to each other by cap material.
Preferably the caps are formed of a material which will etch under an oxygen plasma etch but is substantially unaffected by an etch to remove wafer material.